Plasma gun outmatches the thumper/spinfusor by the rate of fire alone. Therefore indoors you are in a severe disadvantage, as you cannot dodge the hail of so effectively. The projectiles are also a lot harder to see out in the open, giving the raider a distinct advantage. Also, don't the raiders get a shield bonus? So you have to deal with a ridiculously good weapon and way better armor than what you have.
I don't think it is a mere chance that raiders top the scoreboards now.